Is Daylighting Still Cost-Effective in the LED Era?

Twenty years ago, daylighting was regarded as one of the most promising avenues to reduce energy consumption in office buildings—but it never took off. Now, with highly efficient LED lighting taking office buildings by storm, it might seem that the market window for daylighting retrofits has permanently closed.

However, daylight harvesting lighting controls are much less expensive today than they were in the fluorescent era, and the numbers show that they can still be cost-effective in a substantial fraction of office building floor space well into the next decade.

On the other hand, daylight harvesting lighting controls aren’t currently more cost-effective than they were in the fluorescent era, so penetration will likely remain stagnant unless either of two things happens:

• The non-daylighting benefits of smart networked lighting are recognized as being compelling enough to drive deep market penetration on their own.
• Automated shading technology capable of responsive daylight control becomes cost-effective and widely adopted.

Either of these developments would dramatically increase the perceived cost-effectiveness of daylight harvesting, spurring broader use.

The Case for Daylighting

In its broadest sense, daylighting is a strategy for illuminating interior spaces that attempts to maximize the ratio of natural to artificial illumination. Daylighting can provide a healthier, more pleasant, and more productive visual environment while also saving energy.

Daylighting can save energy because it reduces the need for artificial lighting. But to actually realize those savings, the lights have to be dimmed or turned off when less artificial illumination is needed—and that’s called Daylight harvesting.

Daylight harvesting works in any type of building, but is especially attractive in office buildings because they have a relatively high installed Lighting Power Density (LPD) and plenty of windowed area.

Daylight harvesting can be as simple as manually switching off the lights when there is plenty of daylight and switching them on again when there isn’t; unfortunately, that just doesn’t happen in the real world. Therefore, daylight harvesting usually refers to automatic dimming of the lights using a daylight-harvesting lighting control.

Daylight harvesting first became a hot topic in the aftermath of the first energy crisis in the early 1970’s.  Back then, lighting was dominated by relatively inefficient T12 fluorescent bulbs (in commercial buildings) and even less-efficient incandescent bulbs (in residential buildings).  There were two motivations for the interest in daylighting:

• Reduction in lighting energy consumption due to reduced need for artificial illumination.
• Reduction in HVAC energy consumption in warmer climates, due to the fact that daylight has less heat per lumen than the light produced by incandescent or fluorescent lamps.

In fact, studies performed in the 1980’s and 1990’s showed that daylighting could save as much as half of the lighting energy, along with a significant reduction in the HVAC energy, under favorable conditions.  Since about one-third of commercial floor area is close enough to a window to benefit from daylighting, the projected aggregate savings were enormous.

Just as importantly, by the early 1990’s, daylight-harvesting lighting technology was becoming  mature and inexpensive enough for mainstream use.

And yet daylighting never took off.

Daylight Harvesting’s Paltry Market Penetration

The best source of information on the market penetration of various technologies in office buildings is the Commercial Buildings Energy Consumption Survey (CBECS).  The CBECS is conducted periodically by the Energy Information Administration (EIA) and documents energy-related building characteristics and energy usage data for U.S. commercial buildings.

The most recent CBECS was performed in 2018, while the preceding survey was done in 2012.

Here’s a snapshot of some of the data from those two surveys, showing the change in the percentages of the number of office buildings equipped with various lighting technologies from 2012 to 2018:

Daylight harvesting differs from the other technologies in one very important respect: it makes sense only in a building’s perimeter zone (or under a skylight).  Therefore, to provide an apples-to-apples comparison of technology penetration, the Y-axis percentage of Figure 1 is based on the number of  office buildings—and not the aggregate floor area—in which each technology is installed.

Note that daylight harvesting has only a fraction of the penetration of the other technologies, and it’s the only technology whose usage actually seems to have decreased between the two surveys.  That apparent decrease is probably within the margin of error, but it certainly seems that daylight harvesting has plateaued at only about 2 percent.

Compare that to the 20 percent penetration of occupancy sensing.  Occupancy sensing makes for a particularly apt comparison to daylight harvesting because the technologies matured at about the same time, have a comparable energy-saving potential, and have about the same installed cost per square foot.

So why is there an order-of-magnitude difference in the penetration?

Daylighting’s Achilles Heel

Surveys of building operators and other stakeholders have identified the most likely reason that daylighting hasn’t caught on: there is a widespread belief that the actual energy savings often fall far short of the promised savings.  And, like many beliefs, this one turns out to be mostly true.

However, the problem isn’t with daylight-harvesting per se—the problem is that there isn’t enough daylight  to harvest.

Most of the potentially daylit space in U.S. buildings is side-lit with eye-level view windows, and view windows must be shaded to control occasional glare.

Unfortunately, manually operated window coverings are typically adjusted less than once per day, and window coverings on sunny windows are typically left in a mostly closed position. The implication is that they’re adjusted to block glare under worst-case conditions, and then left in the same position even when the risk of glare has abated. This reduces the average amount of glare-free daylight by about 50% relative to what would be possible if the shades were always optimally adjusted.

This might suggest that the savings from daylight harvesting might also be only about 50% of what would be possible without over-shaded windows, and in fact that seems to be true.

According to a 2005 study—perhaps the most comprehensive study ever of the actual, real-world savings provided by daylight-harvesting lighting controls in side-lit spaces—the average relative savings in lighting energy were 3.59 Full-Load-Hours (FLH) in side-lit spaces without manually operated blinds, but only 1.84 FLH in spaces with manually operated blinds (“Sidelighting Photocontrols Field Study” 2005, Appendix F, Table of Full Load Hour Savings, Category “window controls”). Thus, the spaces with blinds were providing only about 50% of the energy savings of the spaces without blinds. Further, these statistics actually imply a much greater savings loss due to over-shading, because only the windows that received little sunlight were likely to be without blinds.

Since it’s not reasonable to expect people to manually adjust their shading throughout the day, the only solution is window shading that self-adjusts to maintain a desired level of glare-free daylight. We call this responsive daylight control and address it thoroughly in a separate post.

Unfortunately, responsive daylight control is far more challenging to implement with today’s technology than daylight harvesting, and its use is virtually non-existent in today’s buildings. The lack of cost-effective responsive daylight control is undoubtedly one of the reasons daylight harvesting never took off.

Has the Window Already Closed for Daylighting?

Another thing in common between occupancy sensing and daylight harvesting is that the absolute savings (in kWh or dollars) depend on the inefficiency of the lamps.  The more efficient the lamp technology, the less the savings from occupancy sensing or daylight harvesting.

This begs an obvious question: if daylighting couldn’t establish a market foothold in the heyday of inefficient T12 fluorescent bulbs, what chance does it have in today’s LED era?  In other words, has the window of market opportunity for daylighting permanently closed?

GSA’s Assessment of Daylight Harvesting Technology

The U.S. Government Services Administration (GSA) administers a Green Proving Ground (GPG) program aimed at evaluating emerging energy-efficient technologies in operating federal buildings; GPG-015 (completed in March 2013) assessed daylight harvesting in five operating federal buildings. The following resources are available at the GPG-015 webpage:

• A 4-page brief summarizing the overall assessment (GPG-015, 2014)
• A detailed technical report (Robinson et al, 2013)

These resources provide the most detailed independent technical information on the cost-effectiveness of daylight harvesting available in the public domain.

Here’s what the GSA said back in 2013 about sites where daylight harvesting (which they refer to as Integrated Daylighting Systems, or IDS) could be expected to be cost-effective:

“…a rule of thumb for sites where IDS should be considered is an installed LPD of higher than 1.1 Watts per square foot and a lighting EUI of approximately 3.3 kilowatt-hour per square foot or higher, although the analysis of site results indicates significant variation on this (one site is estimated to be cost effective at a pre-retrofit EUI of 2.67 kilowatt-hours per square foot). For whole-building modernization and new construction projects, the costs associated with implementing IDS would be lower due to the incremental cost increase of including IDS in an advanced, integrated lighting control system. Furthermore, as the market for advanced controls matures, it is anticipated that the cost of materials and labor will reduce. Consequently, IDS should be considered for implementation in all future new GSA buildings” (Robinson et al, page 102; emphasis on Lighting EUI added for purposes of this post).

GSA’s rule-of-thumb of a Lighting EUI (LEUI) of no less than 3.3 kWh/ft² was based on a target payback period of no less than 10 years, which is still a valid cost-effectiveness threshold for today’s market. However, the installed Lighting Power Density (LPD) and LEUI have decreased significantly since then.

In fact, the market penetration of high-efficiency LED lighting has been so rapid—and it takes so long to survey the characteristics of the building stock—that there’s no way of knowing the exact average current LEUI in office buildings. However, we can make a pretty good guess by extrapolating historical LEUI data from the CBECS.

Trends in Lighting Energy Use Intensity

The following figure shows the office building LEUI from the past three CBECS, along with an exponential curve fit:

The curve fit suggests a 2024 average LEUI of 1.3 kWh/ft², which is only 40% of the 3.3 kWh/ft² cost-effectiveness threshold suggested by GSA. Conversely, it suggests that the average payback period today would be about 10 years divided by 40%, or 25 years—if the costs were the same today as in 2013.

Fortunately, daylight harvesting is considerably less expensive with LED lighting than it was with fluorescent lighting. But before we address costs, let’s also consider the fact that some buildings will have a higher LEUI than the average.

Unfortunately, the 2018 CBECS didn’t provide any information on the distribution of LEUI in office buildings, but it did provide the 25th percentile and 75th percentile (as well as the median) for the overall electricity EUI in office buildings. Fitting a normal distribution to these values yields the following Cumulative Distribution Function (CDF):

If we assume that the LEUI distribution has the same shape (i.e. the same coefficient of variation, which is the ratio of the standard deviation to the mean), we get the following CDFs from the median extrapolated LEUIs shown in Figure 5 for 2024 and 2030:

The Y-axis in Figures 6 and 7 can be interpreted as either the probability that a given building will have an LEUI no greater than the X coordinate, or as the percentage of floorspace across all buildings in which the LEUI will be no greater than the X coordinate.

We’ll come back to these LEUI CDF curves later.

Daylight Harvesting Costs

In their 2013 assessment, GSA said this about the cost at which daylight harvesting becomes cost-effective:

“As a rule of thumb, daylight harvesting becomes cost-effective at an installed cost of $1.40/ft2, with a utility rate equal to the national average of $0.10/kWh, assuming dimming ballasts are already in place.” (GPG-015, page 3)

The national average utility rate isn’t much higher today than it was back then, but daylight harvesting costs have certainly declined.

In 2020, the Northwest Energy Efficiency Alliance (NEEA) commissioned a study by Energy Solutions of Oakland, CA to assess the incremental costs of Luminaire Level Lighting Controls (LLLC). LLLC, as defined by the Energy Solutions, includes not just photocells for daylight harvesting, but also high-end trim, dimming, and occupancy sensing. Energy Solutions defined three levels of LLC as part of their cost survey:

• “Clever” LLLC systems provide high-end trim, dimming, occupancy sensing, and photocells and have “plug and play” fixtures which require little or no additional programming costs upon installation.
• “Smart” LLCL systems include all “clever” capabilities but can also analyze and communicate energy and non-energy data to inform decision-making.
• “Clever-Hybrid” systems that fall between smart and clever: they include a standalone gateway and provide additional functionality such as energy monitoring yet lack the full IoT capabilities of a smart system.

Craig DiLouie (probably the most prolific journalist to ever cover the lighting controls industry) has a blog post which provides the following summary of Energy Solutions’ LLLC incremental cost findings:

• $0.58/ft² for Clever systems
• $1.16/ft² for Smart systems
• $0.78/ft² for Clever-hybrid systems

Note that the $0.58/ft² incremental cost for the Clever systems is only about 40% of the $1.40 threshold for cost-effective daylight harvesting suggested by GSA in 2013.

Projected Daylight Harvesting Payback Periods

Assuming that daylight harvesting costs only 40% as much as it did in 2013, and based on GSA’s 2013 threshold LEUI of 3.3 kWh/ft² for a 10-year payback, we can convert the LEUI CDFs of Figure 7 into payback period CDFs:

These curves suggest that current daylight harvesting technology could provide a payback period of 10 years or less in about 50% percent of U.S. office building floorspace in 2024, and about 20% in 2030. That’s a lot of floorspace, even in 2030.

Further, the potential market penetration of daylight harvesting could be even deeper than suggested by these CDFs, because the “Clever” LLLC systems costed by Energy Solutions provided additional useful capabilities beyond daylight harvesting. This implies that the incremental cost of daylight harvesting is actually lower than the figures quoted above, which in turn implies shorter payback periods for daylight harvesting per se.

While the Window’s Still Open, the Challenges Remain

While Figure 8 suggests that daylight harvesting can still provide a payback period of no greater than 10 years over a substantial amount of floorspace, that 10-year payback isn’t any shorter than it was in 2018 when the CBECS found just a 2% penetration.

So, if daylight harvesting’s market penetration stalled in the fluorescent era, why would it accelerate in the LED era?

It wouldn’t—unless at least one of two things happens:

• Further dramatic reductions in effective price. This could happen if the non-daylighting benefits of smart networked lighting are recognized as being compelling enough to drive deep market penetration; in that case, the incremental cost of adding daylight harvesting could be much lower than the $0.58/ft² assumed in Figure 8.
• Broad use of responsive daylight control technology. This would significantly increase the average amount of glare-free natural illumination in side-lit spaces, potentially doubling the average savings from daylight harvesting. Unfortunately, responsive daylight control technology is currently much more expensive than daylight harvesting. Fortunately, as explained in our dedicated post on responsive daylight control, it also provides other benefits that are actually more valuable than the energy savings from daylight harvesting:
  • It can reduce loads on the HVAC system. In buildings with efficient LED lighting, these HVAC savings can be greater than the lighting savings from daylight harvesting.
  • It can provide a healthier and more productive visual environment. The resulting economic benefits are arguably far more valuable than any energy savings.

Unfortunately, both of these things would require a cultural shift regarding investments in building technologies, namely an increased appreciation of the non-daylighting benefits of (1) smart networked lighting, and (2) responsive daylight control, respectively.

References

Alastair Robinson, Claudine Custodio, and Stephen Selkowitz. “Integrated Daylighting Systems.” Prepared for the U.S. General Services Administration by the Lawrence Berkeley National Laboratory. March 2013. <https://www.gsa.gov/system/files/GPG_IDS_Report_Final_508_Compliant.pdf>

Craig DiLouie. “NEEA Report: Luminaire-Level Lighting Control Costs Decline.” April 2021. <https://lightingcontrolsassociation.org/2021/04/23/luminaire-level-lighting-control-costs-decline/>

General Services Administration, “GPG-015 July 2014: Integrated Daylighting Systems.” <https://www.gsa.gov/system/files/GPG_Findings_015-Integrated_Daylighting.pdf>

Heschong Mahone Group, Inc. “Sidelighting Photocontrols Field Study”. Report prepared for the Northwest Energy Efficiency Alliance, Pacific Gas and Electric Company, and Southern California Edison Company. 2005. <Sidelighting-Photocontrols-Field-Study>

Teddy Kisch and Kate DoVale. “2020 Luminaire Level Lighting Controls Incremental Cost Study.” Report #E21-415, prepared for the Northwest Energy Efficiency Alliance (NEEA) by Energy Solutions. January 7, 2021. <https://neea.org/img/documents/2020-LLLC-Incremental-Cost-Study.pdf>